Conventional plants for the treatment of sewage operated by the activated sludge method operate as follows:
Sewage is supplied in the plant to an aeration basin. Deteriorated products are removed in said basin by a biomass of aerobic microorganisms. These deteriorated products serve as a substrate for the microorganisms. Aerators are present in said basin in order to forward air to the aerobic biomass and to ascertain that said biomass is in a suspended position in the aeration basin. The mixture of biomass and said substrate in the suspended position is called mix liquid suspended solids (M.L.S.S.). The M.L.S.S. is forwarded from the aeration basin to a clarifier wherein the effluent is separated from the sludge. The sludge precipitates and is sucked from the bottom of the clarifier and partly (about 93%) recycled to the entrance of the raw sewage. The effluent leaves the clarifier at its top and is forwarded for agricultural use.
A certain amount of the sludge precipitated at the clarifier leaves the system as excess sludge and is led towards drying beds after it has been subjected to a thickening process in a flotation device in order to enable the biomass in the system to be renewed.
This process will be called hereinafter a method for the treatment of sewage (as herein defined).
The amount of biomass in the plant is proportional to the bio degradable matter which is present in the raw sewage and the possibility to provide air (oxygen) to the plant.
The main drawback of said conventional activated sludge plant is that the biomass in the clarifier does not precipitate well and thus large amounts of untreated sewage i.e., bulking sludge are obtained.
One of the most important indices of the precipitation property of the M.L.S.S. in the clarifier is called SVI which is defined by the following equation: ##EQU1## or, in other words, SVI is equal to the volume of 1 gram of M.L.S.S. after precipitation of 1/2 hour. The SVI under regular conditions in a conventional activated sludge plant is, under certain circumstances, 100-150 SVI, but under special circumstances, 200 SVI.
As the result of said drawback, the operation capability of the activated sludge plant for the treatment of sewages is significantly restricted, thus when SVI is high the separation capability of sludge/effluent in the clarifier is low and therefore the amount of biomass which can be retained in the aeration basin is low. Thus, in case that the amount of organic material is high, there is not enough biomass to decompose it and therefore, together with the excess sludge, there remains living material which has not been subjected to aerobic decomposition. Thus, there begins uncontrolled anaerobic fermentation, which causes unpleasant smells.
A further problem is the "fleeing" of the sludge with the effluent, which spoils the quality of the effluent as the result of bad precipitation of the biomass.
As a result of said drawbacks, one has to reduce the amount of sewage being treated in the system.
There are various means to improve said known methods, inter alia, the following ones:
1. To improve the mixing in the aeration basin in order to avoid unaerobic pockets; PA0 2. to facilitate the movement of the return sludge; PA0 3. to try to improve the precipitation of the biomass; PA0 4. to facilitate the removal of the excess sludge
However, all said solutions are not entirely satisfactory and therefore very often one has to reduce the amount of sewage which has to be treated and/or to enlarge the plant, which is very costly. However, as it is readily understood, said solutions are very unsatisfactory.
It has therefore been desirable to find a solution which would improve the method, i.e. would reduce the SVI and thus enable the treatment of a larger amount of sewage.